1. Field of the Invention
The present invention relates to a method for producing light-scattering elements or an impression template therefor by holographically illuminating a layer of a photosensitive material on a support plate and subsequently developing this layer in order to generate a surface structure.
Light-scattering elements, such as are produced with the present method, are used, in particular, for visualizing images or sequences of images in projection technology. Light-scattering elements in the form of ground glass screens onto which an image is projected are employed in many image representation systems. The image-generating light impinging on the ground glass screen is scattered by it, thus deflected in different directions. This scattering permits recognizing the image projected on the ground glass screen from different directions. For example, scatter screens or respectively ground glass screens are used, for instance, in 35 mm reflex cameras or in medium-size cameras. Rear-projection or front projection systems also require a light-scattering element in the form of a large-size projection picture screen or a ground glass screen onto which the image is projected. The light field leaving the projection screen respectively the ground glass screen is diffuse due to the light scattering thus these light-scattering elements are also called diffusers
A diffuser can either be operated in transmission, for example in rear projection systems or in reflection, for example in front projection systems. In the present patent application, a plate-shaped element respectively a screen composed of a material that scatters the directed incident luminous beam by volume scattering or surface scattering is called a diffuser.
An essential characteristic of diffusers is their scatter profile which shows the angle-resolved scattering efficiency when the incident luminous beam is collimated perpendicular on the diffuser. Scattering efficiency is defined as the relative intensity of the scattering beam measured at a certain angle of emersion normed to the intensity measured at an angle of emersion respectively an angle of view 0. An entire two-dimensional scatter profile comprises the scattering efficiency of the entire half space. Usually, a diffuser can be sufficiently characterized by two one-dimensional scatter profiles in horizontal direction, respectively x-direction and in vertical direction respectively y-direction. A diffuser whose x-scatter profile and y-scatter profile are almost identical is called a symmetrical diffuser. If the x-direction and the y-direction scatter profiles vary greatly, the diffuser is an asymmetrical diffuser.
The scatter profile of a diffuser must be adapted to the intended application. The geometric conditions and the intended optical effect yield a specific viewing angle range for each application. Thus, for large-size projecting screens an asymmetric viewing angle range respectively scattering angle range is usually required in which the horizontal viewing angle should be larger than the vertical viewing angle.
In the ideal case, the scatter profile of a diffuser comprises a rectangular function. In this case, the image inside the specific viewing angle range is always illuminated homogeneously and uniformly bright. On the other hand, no radiation is lost due to scattering in the angle outside the viewing angle range.
However, the diffusers employed in practice usually do not have a rectangular scatter profile but rather a gaussian one. Yet for most applications, it is desirable to be able to produce diffusers with as level as possible scatter profiles, which in the ideal case would come closer to a rectangular profile.
2. Description of the Prior Art
There are various prior art methods of producing diffusers. For instance diffusers, whose function is based on surface scattering, can be provided with a scattering surface structure by treating the surface mechanically, such as for example by sand blasting or by chemical treatment of the surface, for example by means of an etching process. However, these methods usually only permit producing symmetrical diffusers with a gaussian scatter profile. There is no known reliable possible way of selectively influencing the scatter profile.
A possible state-of-the art manner of cost-effective mass production of suited surface structures for the production of diffusers is using moldable materials, such as for example polymers, in conjunction with a replication process. In the replication, using a correspondingly structured impression template, a surface structure is impressed on the polymer body yielding the scattering behavior.
Any desired surface processing method, including the ones described above, can be employed for producing the impression template, which needs to be produced only once. A prerequisite for moldability is that there is no undercutting in the surface structure.
Among other things, the impression template can be produced by illuminating a photoresist layer with a scattered light field and subsequently developing the photoresist layer. The resulting photoresist structure is then transferred to the impression template, for example by means of galvanic molding. The diffusers produced by this method in conjunction with the impression methods reproduce the scatter profile of the irradiated (scattered) light field generated, for its part, with the aid of a diffuser, referred to as the primary diffuser hereinafter. The width of the resulting scatter profile depends on the geometry of the setup. However, the shape of the scatter profile of the produced diffuser depends on the scatter profile of the primary diffuser and is again usually almost gaussian.
State of the art for generating asymmetrical diffusers are, in particular, volume-holography-based illumination methods with which a refraction index modulation is generated in the volume of a light-sensitive layer. The diffusers produced therewith work according to the volume scattering principle.
U.S. Pat. No. 5,365,354 describes a process for producing asymmetrical diffusers in which a light field is passed through a primary diffuser and radiated in a light-sensitive material. A volume hologram is recorded by radiation of the light-sensitive material. With renewed radiation with light, the volume hologram for its part has a scattering effect due to the reconstruction of the recorded primary diffuser. In a further embodiment of the method, two diffuse luminous beams are irradiated from various directions to produce a so-called multiplex diffuser. In a multiplex diffuser, diffusion behavior varies in the different directions, because at a certain angle of incidence only the hologram of one luminous beam is reconstructed and the hologram of the other luminous beam is reconstructed at another angle of incidence. Such a type method is only possible by creating volume holograms and requires that the two wave fields of the two luminous beams are not mutually coherent. However, the method of U.S. Pat. No. 5,365,354 usually leads to gaussian or almost gaussian scatter profiles, because the shape of the scatter profile of the primary diffuser employed in the illumination setup is duplicated.
U.S. Pat. No. 3,708,217 also describes a method for recording a volume hologram for producing asymmetrical diffusers. In this method, an object wave, which was passed through a primary diffuser, is superimposed with a reference wave on a light-sensitive layer to produce a volume hologram. Suited spatial disposal of the primary diffuser in relation to the light-sensitive layer permits generating a strong direction-dependent scatter profile. However, the use of a planar reference wave results in the optical transfer function of the volume hologram having a strongly periodic part, which can lead to undesirable color effects when employing the diffuser. Furthermore, no solution is offered in this printed publication with which a level as possible scatter profile can be realized.
D. J. Schertler et al's Applied Optics 38 No. 2, 1999, pages 291-303 describes a method for generating a leveled scatter profile in which a combination of a one-dimensional phase diffraction grid is employed with a symmetrically scattering diffuser. These two components can either be disposed in series as single components or constructed in the form of the two surfaces of a transparent support material. The diffuser surface is generated using an etching method and the grid surface is generated using a photoresist method. Providing such a type diffuser is, however, very complicated and expensive due to the necessity of producing two components respectively two differently shaped surfaces. Furthermore, often undesirable color effects occur when irradiating white light due to the strictly periodic diffraction grid.
Based on this state of the art, the object of the present invention is to provide a simple method for producing a light-scattering element respectively a diffuser with which a leveled scatter profile can be obtained at least in one dimension.